This invention relates generally to welding apparatus and particularly to a device for accurately shearing, aligning and welding together two sheets of metal, such as CRT shadow mask material.
Many types of thin metal parts are produced by acid etching the thin metal from which the parts are made. For example, shadow masks for use in the picture tubes of color television receivers are made by such an acid etching process. During the production of such shadow masks, the thin metal from which the shadow masks are produced is coated with a photosensitive material. The aperture pattern which is to be etched into the material is then photographically produced on both sides of the coated metal. The unexposed photoresist material is then washed away leaving bare metal at the places where the apertures and peripheries of the shadow masks are to be etched. The material is then passed through an acid etching process in which the bare metal is etched away to produce the aperture pattern and the partially etched periphery of the shadow mask.
During the process of applying the photoresistive material to both sides of the bare metal, the metal is pulled by a series of rollers through tanks where the photoresist material is applied. When a roll of material is depleated, and it is necessary to start another roll through the system, the beginning of the new roll is welded to the tail end of the old roll to avoid the necessity of threading the new roll through the roller system. Problems frequently arise because the welding requires the two sheets to overlap in order to provide an area for the welding. The ends of the rolls usually are not cut square and, therefore, a rather large overlap is needed to ensure that the sheets overlap for their full width. When the photoresist material is applied to both sides of the sheets in the vicinity of the weld, the material enters the space between the sheets. Subsequently, as the welded overlap passes over the rollers, the material between the sheets is squeezed out onto the rollers. The excess material clings to the rollers and causes a nonuniform distribution of the photoresist material on subsequent parts of the sheet. This excess material renders impossible the accurate etching of the metal sheet.
For this reason, there is a need for a device for accurately shearing the ends of the sheets for aligning two sheets and for minimizing the overlap where the welding of the two sheets takes place. The instant invention fulfills this long felt need.